Typically, r-selected species produce many offspring, each of which is unlikely to survive to adulthood, while K-selected species invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.

In unstable or upredictable environments r-selection predominates, as the ability to reproduce quickly is crucial, and there is little advantage in adaptations that permit successful competition with other organisms (since the environment is likely to change again). Traits that are thought to be characteristic of r-selection include: high fecundity; small size; short generation time; and the ability to disperse offspring widely. Organisms whose life history is subject to r-selection are often referred to as "r-strategists" or "r-selected". Organisms with r-selected traits range from bacteria and diatoms, through insects and weeds, to various semelparouscephalopods and mammals.

In stable or predictable environments K-selection predominates, as the ability to compete successfully for limited resources is crucial, and populations of K-selected organisms are typically very constant and close to the maximum that the environment can bear. Traits that are thought to be characteristic of K-selection include: large size; long life span; and the production of fewer offspring that are well cared for. Organisms whose life history is subject to K-selection are often referred to as "K-strategists" or "K-selected". Organisms with K-selected traits include large organisms such as elephants, humans, trees and whales.

It should be noted that, although some organisms are primarily r- or K-strategists, the majority of organisms fall between these two ecological extremes, and may display traits considered characteristic of both ends of the r-K spectrum. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. However, in reproduction, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.